Representations of Reactions
Construct before-and-after reaction particle models with a consistent key and complete inventory.
AP Chemistry Topic 4.3 · CED printed p. 80 / PDF p. 87. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Moles and molarity
- Ionic charges and molecular formulas
- Intermolecular versus intramolecular interactions
- Ideal gas equation with kelvin temperature
A particle diagram is an equation you can count. Decide first what one drawn object represents: an atom, a molecule, an ion, or a repeated solid unit. Keep that key unchanged across the drawing. Connected circles can represent atoms bonded in a molecule; dissolved ions should be separate and labeled with charge. An ionic precipitate is an array, not a collection of isolated neutral molecules. Now use the balanced equation to count which objects disappear, which appear, and which remain in excess. In 2H2 + O2 → 2H2O, a box with five H2 and two O2 can form four H2O and retain one H2. Do not turn the unused molecule into an atom just to fill space. For a physical change, preserve each molecule while changing spacing or arrangement. A model can omit most solvent molecules for clarity, but its legend must say that; otherwise it can look as though an aqueous solution contains no water.
Let's work through it
A particle box contains three Ca2+, six Cl−, two CO32−, and four Na+, all aqueous. Calcium carbonate is assumed to precipitate completely.
- Two carbonate ions allow two CaCO3 solid units; carbonate is limiting in the 1:1 net ionic equation.
- Draw a labeled solid array containing two Ca and two carbonate units. Leave one Ca2+, six Cl−, and four Na+ dispersed in solution.
- Check remaining solution charge: +2 −6 +4 = 0. The solid is neutral. Omitting spectators would make a net-change sketch, but it would not be a complete final particle inventory.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Five H2 and two O2 react completely as far as possible. What remains?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Five H2 and two O2 react completely as far as possible. What remains?
Worked answer and reason
Four H2O form and one H2 remains. There are ten H atoms and four O atoms before and after.
Stoichiometric ratios apply to countable molecules.
Retrieval 2
How should NaCl(aq) be drawn?
Worked answer and reason
Separate Na+ and Cl− ions with equal counts, not bonded NaCl molecules. Indicate water/solvation or state that water is omitted.
The model must reflect species actually present.
Catch the tempting mistake
- Balanced coefficients give the total starting particles.
- Coefficients give the amounts that react. Starting mixtures can contain excess reactant.
- Spectators vanish because they are absent from the net ionic equation.
- Spectators remain in the solution and must appear in a full particle inventory.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 4 problem 5 · ap application · particle
- Unit 4 problem 6 · ap application · particle
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For the teacher
- Have students annotate a before/after atom and charge ledger.
- Reject a drawing that changes the particle key halfway through.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 4.3.A
- Represent a given chemical reaction or physical process with a consistent particulate model.
- 4.3.A.1
- Balanced chemical equations in their various forms can be translated into symbolic particulate representations.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Writing and Balancing Chemical Equations — Scientific check: conserved atoms/charge and complete/net ionic conventions.
- OpenStax Chemistry 2e: Reaction Stoichiometry — Scientific check: mole ratios and limiting reactants; all numerical tasks are newly authored.
Glossary and deeper reading · 6 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Molecular formula
A molecular formula gives the actual number of each kind of atom in a molecule. It can be a whole-number multiple of the empirical formula. For example, glucose has molecular formula C₆H₁₂O₆ and empirical formula CH₂O.
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Ideal gas law
The ideal gas law relates pressure, volume, amount, and absolute temperature in an idealized gas model. It treats particles as having negligible volume and no intermolecular attractions. For example, at fixed amount and temperature, doubling the container volume halves the ideal gas pressure.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Precipitate
A precipitate is a solid phase that forms from species in a solution. Whether it forms depends on the ion concentrations and the relevant equilibrium conditions. For example, silver ions and chloride ions can produce a silver chloride precipitate.